Predistortion and postdistortion techniques, for cancelling the distortion introduced by the nonlinear transfer characteristc of electromagnetic signal devices, are well known in the art. Typically, in such circuits a distortion generator is included in either the input or the output circuit of the device for the purpose of introducing compensating distortion components which serve to cancel the distortion components produced by the signal device. Obviously, the distortion compensating generator must be capable of operating over the same dynamic range and the same frequency band as the device to be compensated. In addition, the relative magnitudes and phases of the compensating components generated should be such as to cancel all the significant distortion components produced by the signal device without itself introducing other spurious distortion components. Finally, all of this must be done at a reasonable cost if it is to have any commercial value. For example, U.S. Pat. No. 3,755,754 dicloses an arrangement for compensating an amplifier employing a velocity modulation tube by means of a second velocity modulation tube or other device that has a distortion characteristic that is substantially similar to that of the amplifier to be corrected. Such an arrangement, however, can be prohibitively expensive if the only way of matching the distortion characteristic of the amplifier is to add a second, like amplifier to the circuit.
U.S. Pat. No. 3,952,260 provides an alternative distortion correcting circuit employing a less expensive diode as the nonlinear element. In this circuit, however, the diode is forward-biased and serves as a nonlinear resistance. Aside from the added loss introduced by the forward-biased diode, there appears to be no means for compensating nonlinear phase distortion. Thus, only partial compensation is possible by this arrangement.
U.S. Pat. No. 3,383,618 similarly employs a diode as a distortion compensating generator. Used in this manner, a relatively large component of the useful signal is required to drive the diode in order to produce the necessary distortion components. A correspondingly large signal component may then interact destructively with the useful signal at the output of the distortion generator thereby reducing the total output signal. In addition, the diode itself, in order to produce amplitude distortion compensating components, must serve as a nonlinear resistor and, thereby, introduces loss to the system.
Thus, the prior art distortion generators tend to be relatively costly and inefficient.